Cold table

ABSTRACT

A ceramic composite is provided comprising ceramic fibers and microparticles bound together as a porous matrix with a ceramic binder. The ceramic composite is particularly useful for transporting cryogenic fluids.

This application is a Divisional of U.S. Ser. No. 08/404,015 filed Mar.13, 1995; which is a Divisional of U.S. Ser. No. 08/124,419 filed Jul.28, 1993, now U.S. Pat. No. 5,441,682; which is a Continuation of U.S.Ser. No. 07/527,600 filed May 23, 1990, now abandoned; which is aContinuation-in-Part of U.S. Ser. No. 07/381,498 filed Jul. 18, 1989,now abandoned; which is a Continuation-in-Part of U.S. Ser. No.06/698,496 filed Feb. 5, 1985, now U.S. Pat. No. 5,041,321; which is aContinuation-in-Part of U.S. Ser. No. 06/667,508 filed Nov. 1, 1984, nowabandoned.

TECHNICAL FIELD

This invention relates generally to fibrous ceramic materials, methodsof their manufacture and methods of use thereof.

BACKGROUND OF THE INVENTION

Our prior application referenced above describes fibrous ceramicinsulation materials formed by dispersing a suspension of ceramic fibersover a form-defining surface, drying the fibers to form a mat,solidifying the mat by soaking it with a sol-gel ceramic precursor, andgelling and curing the sol-gel precursor. The present invention isdirected to an improvement to achieve improved mechanical strength, alower dielectric constant and more isotropic properties as compared tothe fibrous ceramic of our prior application.

SUMMARY OF THE INVENTION

The present invention improves on the fibrous ceramic disclosed in ourprior application by providing a novel ceramic material comprisingceramic fibers and glass microballoons (and/or diatoms), which is setafter impregnation with a sol-gel. The glass microballoons are usuallyabout 5-200 microns in diameter and are hollow. Solid particles may beutilized, however, they increase the density of the ceramic composite.Various glasses with different wall thicknesses (in the case of hollowspheres) can be used to obtain the desired mechanical strength. In placeof, or in combination with microballoons, diatoms may be used. Diatomsare porous silica inner support structures of certain marine and freshwater algae, having typical size ranges of 5-50 microns.

We have found that the rigidized fiber mat made in accordance with ourprior application consists of fibers which are randomly tangled, most ofwhich are oriented in the x-y plane (the z-axis being aligned with thethickness of the mat). When bound with the sol-gel binder, the randomthree dimensional network provides a material, with a porosity of about90-95%. However, according to the present invention, the voids betweenthe ceramic fibers are filled by microballoons and/or diatoms, thusfurther rigidizing the structure. The mechanical compressive strength isincreased as the fibers are supported by the microballoons (and/ordiatoms). The strength of the material approaches that of themicroballoons. Isotropic structural properties (i.e., similar propertiesin the x-y, x-z and y-z planes) are also more closely approximated. Forcomparison, typical useful densities of the fibrous ceramic of our priorapplication are in the range of 15-23 lb/ft³. The useful range of thenovel ceramic composites of the present invention may be tailored to beas low as about 7 lb/ft³. Lower densities, may be achieved by usinglower density microballoons and/or diatoms to achieve an increasedstrength-to-weight ratio. Selected densities may be achieved forspecific applications. The processing time for preparing the ceramicmaterial is substantially reduced, usually by one-half, over the timefor making an all fibrous structure without sacrificing strength.Furthermore, problems associated with handling ammonia (used to cure thesol-gel binder) are reduced. In certain applications, the ceramicaccording to the present invention has a lower dielectric constant thanthat of the fibrous ceramic of our prior application. For example, atmicrowave frequencies, ceramic composites according to our priorapplication have dielectric constants around 1.4, whereas composites ofthe present invention have dielectric constants around 1.2.

Another significant advantage of the improved ceramic according to thepresent invention is its ability to transport and hold fluids (i.e.,wickability), especially cryogenic fluids, such as liquid nitrogen.While not intending to limit the invention to any particular theory, itis believed that the capillaries in the improved ceramic composite allowliquid transport over large distance regardless of the spatialorientation of the required delivery site.

The material of the invention has good thermal shock resistance whichallows for repeated cycling of the cryogenic fluid through the material.It also provides its own thermal insulation for the cryogenic fluid.

The microparticle enhanced fibrous ceramic (hereinafter sometimesabbreviated as MEFC) material according to the present invention is thusmade by forming a slurry comprising ceramic fibers and microparticles,preferably microballoons and/or diatoms, in a liquid; dispersing theslurry over a form-defining surface, removing the liquid to form a wetmat of fibers and microparticles; drying the mat; soaking the mat with asol-gel ceramic precursor binder; and gelling and curing the binder andmat at a sufficient temperature and for a sufficient period of time toconvert the sol-gel precursor to ceramic, thereby forming themicroparticle enhanced fibrous ceramic.

The microparticles are preferably glass microballoons and/or diatomswhich are hollow, but solid microparticles may also be used.Microballoons have a crush strength in the range of about 500 to 4500psi and are commercially available. The sol-gel binder is preferably analumina precursor, but other sol-gel binders such as sol-gel precursorsfor silica, mullite, yttria, zirconia, boroalumina-silica, or any otherof the sol-gels known to those skilled in the art may be used. Additivessuch as single crystal whiskers (silicon carbide, silicon nitride,alumina, etc.) may be used to impart desirable properties to the MEFC,such as to increase compressive strength and modulus, mechanicalstrength, and bulk porosity, etc.

A particularly preferred application of the MEFC is for preparation of acryogenically cooled surface. The MEFC maybe coated for example, bysputtering, plasma deposition, or application of paint, with a metal ormetal oxide superconducting composition, or a precursor thereof. Asuperconductive film may be utilized, for example, as an antenna orelectromagnetic shielding.

If the MEFC is coated with glass on the surface and the glass ispolished, the resulting composite can serve as an optical surface tosupport a mirror.

A particularly preferred method of use of the MEFC is for transporting acryogenic refrigerant to and/or from a locus to be cooled whereby therefrigerant is transferred through the porous MEFC. Many other uses ofthe MEFC are also possible, including use as a Dewar stopper, use as acryogenic work table or a sample holder for working in a coolenvironment (for preparing frozen sections for histology), or use as abulkhead, firewall or heat pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an electronics package cooled by the MEFCmaterial of the present invention attached to the exterior of thepackage.

FIG. 2 is a schematic cross-section of a Dewar with a stopper of MEFCmaterial.

FIG. 3 is a cross-section of a Dewar with a stopper of MEFC material,with an optional working surface which serves as a portable freezer.

FIG. 4 is an illustration of a work surface cooled by MEFC material in atray of cryogenic fluid.

FIG. 5 is a schematic cross-section of a refrigeration unit made of MEFCmaterial in communication with a cryogenic fluid.

FIG. 6 is a section of a heat pipe comprising an inner wick of MEFCmaterial.

DETAILED DESCRIPTION OF THE INVENTION

One component of the MEFC according to the present invention is ceramicfiber. Ceramic fibers are known and many are commercially available.Preferably the ceramic fibers will be alumino-silicate fibers, but otherfibers may be utilized including, but not limited to, fibers of silica,alumina, boroaluminasilica (commercially available under the trademarkUltrafiber® 440 from 3-M Company), zirconia, silicon nitride, andmixtures thereof. The fibers are available in various dimensions,usually from about 0.3 to 4 inches in length (the longest dimension ofthe fiber) and 1 to 10 microns in diameter. It should be realized thatthe dimensions of the fibers can be tailored to meet the physicalcharacteristics which are desired in terms of mechanical strength, etc.

The glass microballoons are commercially available in many sizes and aregenerally hollow spheres made from various types of glass compositionswith various wall thicknesses, usually in the size of about 5 to 200microns in diameter. Solid spheres or diatoms may also be used in placeof or in addition to the hollow microballoons. As in the case of thefibers, the size of the microballoons will in part determine themechanical strength and physical characteristics of the MEFC.Preferably, the microballoons should be in a range of about 5 to 50microns, which appears to be the preferred size for filling the voidswhich would otherwise exist between the fibers, thus increasing thestrength of the MEFC. In some cases the MEFC may consist entirely ofmicroballoons.

Typically, an aqueous slurry of the ceramic fibers and microballoonsand/or diatoms is mixed to provide a substantially uniform dispersion.The concentration of the slurry is not particularly critical but, forconvenience, the slurry will generally comprise up to 10 wt/% of thefibers, up to 10 wt/% of the microballoons and/or diatoms (based on thetotal weight of the slurry) with the remainder being water. The slurrymay contain from 0-99 wt % of ceramic fibers and from 1-100 wt %microparticles, based on the combined weight of fibers andmicroparticles.

Diatoms may be used in the slurry. The extremely fine porosity of thediatoms may increase mechanical strength while also increasing the bulkporosity, which allows for higher cryogen incorporation per volume andsmaller, more controlled porosity for gas/liquid interface control formembrane applications. The diatoms may comprise up to 100% by weight ofthe combined weight of the fiber and microparticles (microballoons anddiatoms).

Typically, after the slurry has been thoroughly mixed, it is poured(i.e., vacuum-deposited) over a form-defining mold (usually porousenough to allow passage of the water therethrough but not the fibers ormicroballoons) which may be flat, irregular, curved, or virtually of anysize or shape. The water is then removed typically by vacuum through theporous mold thereby forming a wet mat. The mat is then dried, usually ata temperature of up to about 200° F., to completely remove the water.Duration of the drying will, of course, depend upon the size and shapeof the mat.

Once the mat has been dried, a sol-gel binder is introduced, preferablyin incremental stages. The binder is preferably an alumina sol-gel glassthat can be prepared by known techniques, such as those disclosed in ourprior application. Incremental addition of the binder involves repeatingthe steps of impregnating the mat with the binder, gelling the binderand curing the mat and binder. Usually a light coating of binder isapplied in the first stage followed by an air dried gellation todimensionally stabilize the fiber mat. Thereafter, the steps ofimpregnating, gelling and curing are repeated one or more times untilthe total desired amount of binder has been added. Typically about 15 to300 wt % of binder is used based on the initial weight of fibers andmicroballoons in the mat. The impregnating step may be accomplished bywicking, spraying, vacuum infiltrating, and the like.

After impregnation, the binder is converted to a rigid gel, usually byair drying or by subjecting the binder-impregnated mat to an atmosphereof ammonia gas. Since the ammonia sol reaction is exothermic, thetendency of bubbles to form in the mat can be avoided by allowing thefirst batch of binder to gel in air.

After gelling the binder, the mat is cured, preferably by heating toabout 200° F. for several hours (about four hours are preferred), thenby slowly increasing the temperature to about 600° F. for a longerperiod of time (usually about five hours), and finally by reducing thetemperature to ambient temperature.

In addition to the ceramic fibers and microballoons and/or diatoms, theslurry may also contain additives which can alter the physicalcharacteristics of the MEFC. For example, by adding small ceramicwhiskers (about 0.4 to 1 micron in diameter, 100:1 aspect ratio average)in small amounts (usually about 5 to 30% by combined weight of thefibers and microparticles), the compressive strength and modulus of theMEFC may be increased. Preferably, whiskers of silicon carbide of about0.4 to 1 micron in diameter are useful for this purpose.

The MEFC may, if desired, be coated with a substrate. For example, theMEFC may be coated with a layer of mixed metal oxides by methods such asplasma spraying, sputtering, or by painting. Such a material coated witha mixed metal oxide superconductor layer may be useful, for-example, asan antenna or as electromagnetic shielding enclosures for electronics.

The MEFC may also be coated with glass. For example, this may beaccomplished by brushing a slurry of Pyrex® glass powder (preferably-325 mesh) and water on its surface, drying, preferably in air, in anoven at about 160° F., and firing the composite for several minutes, atabout 2000° F. Other methods of applying glass to the surface includefusing the glass to the MEFC surface by torch, plasma spray, laserrastering, etc.

A high impact-resistant surface may be applied as a slurry to MEFC'ssurface by adding about 5 to 60 wt % Pyrex® glass powder to a matrixmaterial comprising colloidal silica, mullite powder, single crystalwhiskers (such as silicon carbide or silicon nitride) and trona. Thecoating may be dried and fired, usually for about ten minutes, asdescribed above.

Sputtered metal or metal oxide on the glass surface, which is firstpolished to obtain the requisite smoothness, will form a mirror usefulin light weight telescopes, antennas, reflectors, and the like.

The MEFC may also be used as a conduit for carrying a cryogenic coolingagent to a particular site. For example, there are many electronic andoptical devices which require cryogenic cooling such as cold cameras(for astronomical applications), SQUIDS (superconducting quantuminterference devices), bolometers, IR detectors, X-ray equipment,computers, circuit boards and high density packaged electronics. Forexample, the MEFC may be affixed to one surface of a substrate and theother surface of the substrate may accommodate the camera or electronicdevice to be cooled. Other applications of the MEFC include use as acryogenically-fed, cold table with embedded delivery lines. A particularapplication of this cold table is a histology freezer for freezing orcooling tissue samples. Since it is a refractory material, the MEFC maybe sterilized by high temperature methods without structural damage.

Another application is use as a porous explosion-proof stopper for aDewar to relieve pressure caused by boil-off of the cryogen.

An all-ceramic heat pipe is also provided in which there is an MEFCinner wick with a central vapor channel, surrounded by a glass-coatedouter ceramic matrix shell. This heat pipe can be used to coolelectronics or aerospace vehicles. Working fluids include liquidnitrogen, ammonia, water and Freon®.

The MEFC may also be used as a light weight, heat dissipating component,such as use as a non-flammable interior, a building bulkhead (forexample, in extraterrestrial habitat interiors) or engine firewall. TheMEFC can replace conventional organic materials which are toxic whenthey degrade) in reusable thermal protection systems in spacecraft,aircraft, high rise buildings, and automobiles. The MEFC is advantageousin that it does not exude toxic gases, has a lower density than mostorganic materials, and has low thermal conductivity (i.e., is a thermalinsulator).

Referring to the figures, FIG. 1 shows a container of packagedelectronics 10 which is cooled by attachment of a plurality of blocks 11of MEFC material to the outer surfaces of the package 10. The cryogenicfluid (not shown) is in communication with the blocks 11 to cool theelectronics (not shown) within the package 10.

FIG. 2 shows a cross-section of a Dewar comprising a shell 13 having anopening which is sealed by a stopper 14 made of MEFC material. Theporosity of the MEFC allows for gas to escape from within the Dewar,thus relieving pressure. The stopper has an extension 15 which extendsinto the cryogenic fluid 16.

FIG. 3 shows a Dewar and MEFC stopper similar to FIG. 2. The stopper isadapted with a work surface 20 which is cooled by virtue of the wickingof the cryogen. The work surface 20 may be made of metal, glass,ceramics, or other suitable material. The surface 20 may be utilized asa portable histology freezer for freezing and examining samples.

Referring to FIG. 4, a cryogenically-cooled work surface is provided byinserting a slab of MEFC 26 into a tray 27 containing a cryogenic fluid28.

Referring to FIG. 5, there is shown a Dewar with a specially adaptedMEFC stopper 31 which serves as a low cost refrigeration unit. The MEFCstopper 31 includes a cavity 32 which can be enclosed with door 33.Objects to be refrigerated or frozen may be placed within the cavity 32.

Referring to FIG. 6, a heat pipe 40 is shown comprising an MEFC innerwick 41 with a central vapor channel 42 and a ring of arteries 43. Thewick 41 is enclosed by a reinforced ceramic laminate 44 made, forexample, of a ceramic disclosed in copending, commonly assigned U.S.application Ser. No. 07/212,397, filed Jun. 27, 1988, which isincorporated by reference herein. The laminate 44 is coated by Pyrex® orother shock-resistant glass coating 45 to make the heat pipe gas tight.

In all of the above configurations the surfaces of the MEFC which wouldotherwise be exposed to the atmosphere may be coated with a suitablecoating, as glass, to contain the cryogen within the MEFC.

While preferred embodiments have been shown and described, those skilledin the art will recognize modifications, variations, or alternativesthat can be made without departing from the invention. The examples areprovided to illustrate the invention and are not meant to limit it.Therefore, the specification and claims should be interpreted broadly toprotect the invention here described. The claims should be limited onlyas is necessary in light of the pertinent prior art.

What is claimed is:
 1. A cold table for cooling samples on a worksurface, comprising(a) a body defining the work surface of the table,the body being formed from a porous ceramic composite; and (b) areservoir of cryogenic fluid for supplying cryogenic fluid to the worksurface through the porous ceramic.
 2. The cold table of claim 1 whereinthe ceramic composite comprises a felted mat of about 0-98 wt. % ceramicfibers and about 1-100 wt. % microparticles.
 3. The cold table of claim1 wherein the reservoir is a dewar having an open throat and wherein thebody includes a plug that seats in the throat of the dewar.
 4. A coldtable for cooling a sample on a work surface, comprising:(a) a memberdefining the work surface; (b) a porous ceramic composite body incontact with the member for transporting cryogenic fluid from areservoir to the member to cool the work surface; and (c) a reservoirfor cryogenic fluid in contact with the body.
 5. The cold table of claim4 wherein the member is metal, glass, or ceramic.
 6. The cold table ofclaim 4 wherein the member has a cupped portion positioned beneath thework surface and wherein the body has a head portion that seats in thecupped portion of the member.
 7. The cold table of claim 6 wherein thereservoir is a dewar having an open throat and wherein the body includesa plug that seats in the throat of the dewar.
 8. The cold table of claim4 wherein the body is a felted mat of about 0-99 wt. % ceramic fibersand about 1-100 wt. % microparticles.